WebVTT should allow the `transition:` property to apply to cues

[jernoble]

Expressive captioning makes use of animations during :past and :future transitions to convey emotional valence and and intensity. It would be relatively straightforward to apply these transitions to specific timestamps using existing support for CSS so long as transition: was an allowed property.

[nigelmegitt]

Animated per-word transitions appear to be a feature of the "Caption with Intention" design system specifically. As far as I can tell there isn't yet a widely accepted definition of "Expressive captioning" or its technical requirements. It's good to lodge this issue as a placeholder so we don't forget: it seems to me to be a bit early to draft spec text for it though. I'd appreciate stronger signals that this is a desirable (and not harmful) feature.

I've added the labels VTT-next, addition/proposal and needs implementer interest - feel free to adjust those though if they don't feel right.

[jernoble]

Oh! This may be a literal non-issue:

8.2.1. The ::cue pseudo-element
The following properties apply to the ::cue() pseudo-element with an argument:
...
properties relating to the transition and animation features

It may just be the case that this is an underrepresented area in the WPT tests, which would make this an Interop problem, not a spec problem.

[gkatsev]

This actually brings up a point. There's a different list of allowed properties for ::cue and ::cue(). Should that be the case?